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See the PS1 DR2 caveats old page for more information.
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However, the positions of the mean objects in the DR1 release catalog were recalibrated using Gaia DR1 positions as additional constrains in the astrometric solution. The Gaia measurements were given very high weight, as detailed in Magnier et al. (2016). A comparison of Gaia and PanSTARRS positions for the objects in common yields a typical residual of 5 mas (1-sigma, 2-d), with a slightly higher component in the direction of right ascension. This comparison may not fully reflect PanSTARRS position errors, because of the correlation in the quantities that were compared (Gaia measurements are included in the determination of individual PanSTARRS positions).
A side effect of the inclusion of Gaia DR1 positional data is that the epochMean column in the ObjectThin table, which gives the mean epoch for the RA and Dec measurements, is also affected for objects that included Gaia DR1 data in their position calculations. The result is that the epochMean date is often later than any of the PS1 measurements given in the Detection table. Typically the value for objects that include Gaia DR1 data is close to the Gaia DR1 epoch of 2015.0 = MJD 15023. Note that in these cases, the epochMean value is completely unrelated to the mean date for the PS1 photometric measurements.
An independent test of sources with Gaia data that were not used in the recalibration was carried out by Makarov and collaborators (V. Makarov, C. T. Berghea, & J. H. Frouard, Technical Memorandum, AADD USNO, 2017). They consider the over 19 million stars for which the duplicated_source flag is set in the Gaia DR1 catalog for which a Gaia-corrected position is available in PanSTARRS DR1. Their results, summarized in the first plot below, suggests that the residual systematic uncertainties for recalibrated PanSTARRS positions is closer to 20 mas (1-sigma, 2-d) for sources with Gaia magnitudes between G=15 and G=18, increasing for fainter magnitudes to about 35 mas at the Gaia magnitude limit (G=21.7). Errors are larger towards brighter magnitudes as well, possibly because saturation and proper motion effects become significant in the PanSTARRS measurements.
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We hope eventually to fix this issue (and some other problems described in the "FITS image format quirks" section of the PS1 DR2 caveats old page), but since the PS1 archive includes 1.5 petabytes of PS1 images, the task of updating all the image files is not simple.
